Extracellular polymeric substances (EPSs) are involved in both detrimental and beneficial consequences of microbial aggregates such as biofilms, flocs and biological sludges. In biofouling, they are responsible for the increase of friction resistance, change of surface properties such as hydrophobicity, roughness, colour, etc. In biocorrosion of metals they are involved by their ability to bind metal ions. In bioweathering, they contribute by their complexing properties to the dissolution of minerals. The EPSs represent a sorption site for pollutants such as heavy metal ions and organic molecules. This can lead to a burden in wastewater sludge; on the other hand, the sorption properties can be used for water purification. Other biotechnological uses of EPS exploit their contribution to viscosity, e.g., in food, paints and oil-drilling 'muds'; their hydrating properties are also used in cosmetics and pharmaceuticals. Furthermore, EPSs may have potential uses as biosurfactants, e.g., in tertiary oil production, and as biological glue. EPSs are an interesting component of all biofilm systems and still hold a large biotechnological potential.